Lead coating process



United States Patent LEAD COATING PROCESS No Drawing. Application June 13, 1952, Serial No. 293,421

7 Claims. (Cl. 117-130) This invention relates as indicated to a novel lead coating process and material, and more particularly to protective coatings for metal surfaces subject to corrosion, especially iron, steel and ferrous alloys.

The tendency of the less noble metals to revert to their naturally occurring states has been recognized since their first employment, and many different methods have been adopted in an attempt to prevent, arrest or inhibit such corrosion or rusting. It has been estimated that the annual monetary loss due to corrosion in the United States alone is in excess of $5,500,000,000. Of course, indirect losses due to escape of such materials as oil, water, natural gas and other fluids from corroded pipe lines and losses due to down time in transportation facilities and manufacturing plants are difiicult to appraise accurately. It should also be appreciated that construction members and processing equipment, for example, are regularly over-designed to compensate in part for anticipated corrosion. Very large industries have developed providing methods and materials for protecting metal surfaces, including paints, varnishes, lacquers, phosphate coatings, tin plating, galvanizing, and other plating procedures such as the electroplating of zinc, cadmium, copper and nickel. Cathodic protection of pipe lines and the like has recently been developed on a large scale. Obviously, in view of the tremendous quantity of ferrous metals in use, any method of protecting the same even slightly more effectively than methods currently employed would result in a comparable extremely large annual dollar saving.

Each of the known methods of protecting ferrous metal surfaces has certain inherent deficiencies and each such method will ordinarily be employed only in certain selected fields of use. Protective organic coatings, which include the paints and lacquers, are reasonably effective as long as the protective film is not broken, but they tend to be rather expensive and often their cost is justified on the basis of their decorative qualities equally with their protective qualities. The application of metallic coatings both by hot dipping and electroplating is also an old art. The less noble metals such as zinc are generally anodic with respect to iron in most environments and are classified as sacrificial. The protective coating gradually corrodes away and affords considerable protection as long as a continuous film remains. There are situations, however, where zinc as well as lead, tin, cadmium and aluminum will reverse its potential with respect to iron and consequently the position in the electromotive series cannot be wholly relied upon as an infallible index for predicting the galvanic behavior of metallic couples in all environments. Coatings of non-sacrificial metals such as nickel and copper may also form galvanic cells with iron although the current flow is in the opposite direction to that in the case of the sacrificial metals, resulting in accelerated corrosion of the base metal wherever the coating is broken or porous. Relatively thick coatings of such non-sacrificial metals are accordingly commonly employed.

2,813,805 Patented Nov. 19, 1957 .Hot dip methods of applying protective metal coatings have certain advantages over similar coatings electrodeposited but also display certain inherent weaknesses. Thus, hot dip galvanized steel may develop cracks when flexed due to the formation of a hard zinc-iron alloy which is brittle and has poor adhesive properties. The hot dip method and the electroplating method both tend to prof duce coatings rather non-uniform in thickness.

It is accordingly a principal object of the present invention to provide a protective lead coating for metal surfaces subject to corrosion, particulafly iron, steel and the ferrous alloys.

Another object is to provide such coating which will be considerably less expensive than other coatings commonly employed for a like purpose.

A further object is to provide such coating and method of application of the same which will be adapted to substantially all sizes and shapes of articles to be protected and will afford a predictable uniform degree of protection.

Still another object is to provide such coating which will not interfere with subsequent working of the article or the application of other decorative coatings and the like but instead will facilitate the same.

Other objects of this invention will appear as the desciption proceeds.

To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

Our invention comprises the provision of a lead coating on the metal surface to be protected by chemical displacement from a bath containing the lead ion in solution. More particularly, we have found that a remarkably effective lead coating may be deposited on ferrous surfaces from a bath having a pH less than 7 and containing one or more aliphatic poly-basic hydroxy acids.

The aliphatic poly-basic hydroxy acids such as malic, tartaric, citric, maleic and di-glycolic acid form lead salts which are soluble in water to varying degrees. While increased solubility may be achieved by increasing the free acid content of the solution, we have found the resultant baths relatively unsatisfactory for our purpose and instead employ ammonium or alkali metal salts of these same acids to achieve such increased solubility and the desired lead ion concentration. While the lead ion can be obtained from any lead salt soluble in water such as lead nitrate, or lead salts solubilized in acids, it has been found more economical to employ lead oxide (litharge) which combines readily with the aliphatic poly-basic hydroxy acids. I

The ferrous article to be coated may be immersed in the water solution of the lead salt of the aliphatic polybasic hydroxy acid, such as lead citrate, for a period of time suflicient to deposit a lead coating of desired thickness on such article by a chemical displacement reaction. As indicated above, the pH of the solution should be less than 7 and the solution-will also desirably be heated to obtain best results. Instead of immersing the article in-such solution or bath, the article may be sprayed or flow-coated with the solution.

- Addition agents While a usable lead protective coating can be obtained by carefully following the teachings of this invention without the employment of special addition agents, much less care need be exercised when addition agents of the type commonly. added to electroplatingbaths for the purpose of ensuring sound metallic deposits are employed. Typical addition agents include: carbohydrates such as the simple sugars, dextrose, maltose, sucrose and lactose; and the more complex carbohydrates such as dextrin and the starches were found to function satisfactorily. Both animal and vegetable proteins such as casein, bone glue, albumin, gelatin and zein served equally well. Some organic acids such as tannic, pyrogallic, salicylic and gallic also produced the desired effect as addition agents. Special classes of chemical compounds such as aldehydes, furfurals, phenols, cresols, aloin, goulac, licorice, resorcinols, naphthols, gums, and amines served as satisfactory addition agents. The concentration of addition agent in all cases was very small, usually in the range of 0.1 to 1 gram per liter of solution. Greater or less amounts could be used, but in most cases, the concentrations indicated were preferred for commercial operation.

Since some impurities such as traces of certain sugars commonly found in commercial grades of acids contemplated herein may themselves serve the purpose of addition agents, such less expensive grades may be preferred to. chemically pure acids although a supplemental addition agent will also ordinarily be added. Of course, contaminants which would interfere with the displacement reaction or deleteriously affect the coating will be avoided. The precise manner in which such addition agents operate to achieve the improved results has never been wholly understood in the electroplating art and is equally mysterious here. Their presence, however, definitely permits the obtaining of satisfactory coatings throughout a wider range of operating conditions, serving to inhibit grain crystallization.

General operation We have, found that for best results the pH of the solution should be maintained between approximately 4.5 and approximately 6.5, and desirably between 5.4 and 6.0. Best results obtained with typical lead salts of aliphatic poly-basic hydroxy acids, notably lead citrate, were achieved when the pH was maintained in a relatively narrow range of from 5.6 to 5.8.

The rate of deposition of the lead coating is considerably increased by raising the temperature of the solution, and we accordingly ordinarily prefer to maintain such solution at an operating temperatureof from about 130 to. about boiling. A temperature in excess of 160 F. will ordinarily be employed in commercial operations and there is seldom any substantial advantage in exceeding a temperature of'2lO'F. However, usable l'eaddeposits can be obtained at temperatures ranging from about 70 F. to boiling.

The maximum rate of deposition of a high quality lead coating is obtained when the solution is heated as indicated and the pH maintained within the preferred range. A considerable increase in rate can also be effected by increasing the lead ion concentrationwhich will ordinarily be from about 3 to about 50 grams per liter. The usual preferred operating range will be from about to about grams per liter of lead ion to obtain excellent results at reasonable cost. We have found that when a bath has been in use for a considerableperiod of time, peak performance is ordinarily obtained at a pH offrom about 5.4 to about 5.6;whereas in a freshly prepared solution maximum performance is achieved at a pH of about 5.8. This difference may be due to the fact that a' certain amount of iron has gone into solution as a result of the chemical displacement reaction.

For purposes of illustration, the lead citrate bath will be considered below in some detail since this lead solution is relatively inexpensive and affords superior results under properly controlled conditions.

Lead oxide (PbO) may be reactedwith citric. acid to form the lead citrate which is aslightly soluble fine white product readily solubilized in: a-watersolution cone taining an alkali metal or amonium salt of citric acid. Of course, alkali metal and ammonium salts of other aliphatic poly-basic hydroxy acids may instead be employed, and it will be understood that in practicing our invention mixtures of the various aliphatic poly-basic hydroxy acids are entirely suitable. Utilizing sodium citrate as the solubilizing agent in a solution maintained at a temperature of 190 F. and a pH of 5.65, it was found that a solution containing the equivalent of 10 grams per liter of lead ion would require a minimum concentration of about 50 grams per liter of the sodium citrate, and a solution having a lead ion concentration of 20 grams per liter. would require a minimum concentration of about grams per liter of the sodium citrate. When the concentration of the ammonium or alkali metal salt drops below such minimum, lead citrate will precipitate, and the rate of deposition of the lead coating on the ferrous article immersed in the bath becomes reduced. Moreover, such deposit tends to become undesirably hard. It has also been found that if the concentration of the ammonium or alkali metal salts employed as the solubilizing agent is excessive, the rate of deposition of the lead coating is reduced. The exact range for best results of each solubilizing agent varies somewhat depending upon the agent employed, the temperature of the solution, etc. It may be noted that such solubilizing agents also serve the additional function of buffering agents and assist in maintaining the pH in the proper range.

The concentration of addition agent employed may vary from about ,4 gram per liter upwards. The presence of tri-valent elements, such as aluminum and ferric iron and of ammonium or alkali fluorides or bi-fluorides, was also found to improve the adhesion, density and general appearance of the lead deposit.

It was found that from a standpoint of operational economy and efiiciency a concentration of approximately 15 grams per liter of lead ion and about grams per liter of sodium citrate was most desirable. About one gram per liter of animal bone glue was incorporated as an addition agent although considerable fluctuation either way was found to be permissible.

While excellent results Were obtained from the employment of the above-described solution, the quality of the lead deposit eventually was adversely affected by the accumulation of iron in the coating solution resulting from the chemical displacement reaction. When a concentration of about 20 grams per liter of ferrous iron or of about 3 grams per liter of ferric iron was reached, the lead deposit thereafter. obtained. had relatively poor adhesion to the base metal, was somewhat powdery, and had relatively poor malleability and ductility. In order that the plating bath might be operated substantially continuously and maintained in a condition to afford the highest quality of lead coating, it was obvious that some means should be found for coping with this iron problem. The manner of controlling the iron ion concentration in the platingbath is not a part ofthe present invention but is described and claimed in the application of Fred B. Kendall and James R. Kusa for U. S. Letters Patent entitled, Ir on Ion Control, filed even date herewith. It will, however,,be briefly explained below.

It was found that ammonium and alkali metal fluorides and bi-fluorides would selectively precipitate iron in the ferric state without disturbing the ion concentration. Accordingly, it'was decided to oxidize the ferrous iron to the ferric state whereupon it will combine with the fluoride in a complex manner to form a precipitate of ammonium or. alkali metal ferric fluoride under the free acid conditions prevailing. In order to' reform citric acid from the citrate combined with the ferric iron, the bi-fluorides were/preferred to the fluorides, hydrogen being thus made available for combining with the. citrate ion, giving the effect of increased acid concentration. By adding lead oxide as the lead ion concentration is gradually reduced, it will effectively combine with the free acidand the initial concentration of the solution may be maintained. Only sufiicient lead oxide is added at any one time to adjust the pH of the solution to its original value and thereby maintain equilibrium. Moreover, when the ferrous citrate becomes oxidized to ferric citrate, sodium hydroxide is apparently formed which combines with the reformed citric acid to replenish the sodium citrate used during the oxidation stage. It Was found experimentally that the solution would tolerate considerable quantities of the bi-fluoride without disturbing the lead ion concentration. Concentrations of the bi-fiuoride in excess of 20 grams per liter, however, are considered undesirable as tending somewhat to lower the rate of lead deposition and causing such deposit to be relatively hard.

While air alone may be employed to oxidize the iron to the ferric state, the employment of molecular oxygen as a gas or atomic oxygen obtainable from oxidizing salts such as ammonium or alkali metal nitrates and nitrites may sometimes be desirable.

The above-explained process functions equally well in the case of the other aliphatic poly-basic hydroxy acids. Furthermore, it should be understood that the ammonium or alkali metal salt employed need not necessarily be a salt of the same acid as the lead salt but may be a salt of any other water soluble aliphatic poly-basic hydroxy acid. For example, lead citrate may be solubilized in ammonium or alkali metal tartrates, or lead tartrate may be solubilized in ammonium or alkali metal citrates, maleates, etc.

In general, it was found that somewhat thicker lead deposits were obtained in a 15-minute period if the pH of the solution was approximately 5.8 and that thereafter maximum deposition would be obtained at a pH of approximately 5.6. This phenomenon is not entirely understood but appears to hold true throughout the range of effective lead ion concentration. In general, the rate of deposition and thickness of the deposit will increase as the lead ion concentration is increased. An increase in lead ion concentration from 5 to 15 grams per liter resulted in an increase in rate of deposition of approximately 100%. A further increase in lead ion concentration resulted in a further increase in rate of deposition but not to the same degree. There is accordingly no commercial advantage in employing extremely high lead ion concentrations. It has been found experimentally that solutions of lead salts of aliphatic poly-basic hydroxy acids are extremely efficient as the amount of lead actually deposited corresponds very closely to the theoretical value. In fact, any deviation noted was within the range of experimental error.

The following examples of certain highly successful formulations are given by way of illustration and not by way of limitation. They show the use of certain preferred compounds in accordance with our invention.

GENERAL FORMULATION NaaC5H5O7 Addll. A Pb' Cone, Gms./L. NHiFHF 00110., Cone, Cone, Gm./L Gms./L. Gms./L (Min.)

Min. Max

5 15 75 5 to 20 1 t0 1 1O 50 135 5 to 20 0 l to 1 20 125. 245 5 to 20 0 1 t0 1 30 215 340 5 to 20 0 1 t0 1 40 265 420 5 to 20 0 1 t0 1 50 275 440 5 to 20 PREFERRED sonu'rron FORMULATION."

Chemical Components GmsJL. 0z./Gal. Lasfitlo a Lead ion (from PbO)... 15 12. 5 Sodium Citrate 112. 5 Ammonium Bitiuoride. 10 1.33 8. 3 Amber glue 1 4 Gms./Gal.... 0. 8

SOLUTION MAKE-UP USING CITRIC ACID AND SODIUM HYDROXIDE Chemical Components Gms./L. Lbs/100 Gals.

Sodium Hydroxide Citric Acid Lead Oxide (mono) Ammonium Blfluoride- Amber bone glue wee-ammo:

In the above example, it will be noted that lead oxide is used as the source of lead ion because of its economy and availability. Likewise, it is cheaper to prepare the alkali metal salt of the acid from the acid and alkali metal hydroxide than to buy the manufactured salt.

Solution control When the solution has been made up to specified concentration, control to maintain its strength and keep it in continuous operation is relatively simple. The lead ion concentration and the alkali metal or ammonium salt concentration should be maintained within the ranges affording maximum rate of deposition of a high quality lead coating. This may be accomplished by periodically analyzing the solution for lead. It will occasionally be necessary to replenish the bi-fluoride to control the iron concentration, such iron concentration likewise being periodically determined by analysis. As in the case of electroplating baths, the addition agent should be replenished from time to time as the occasion demands.

As explained above, the iron going into solution as a result of lead reduction is oxidized to the ferric state and combines with the bi-fluoride present and is automatically precipitated. The ferrous and ferric ion concentrations may be maintained at almost any point found convenient, but it is most economical to allow the ferrous ion concentration to increase to a maximum of about 8 to 10 grams per liter. At this concentration, oxidation by air is quite efficient to oxidize the ferrous to ferric iron and ordinarily little, if any, additional oxidizing agent need be added. Practically all of the ferric iron reacts with the bi-fluoride (or fluoride) to produce an insoluble compound which readily precipitates and settles to the bottom of the tank and away from the field of chemical reaction. Periodically the solution may be filtered or decanted and the precipitate removed.

While the quality of the deposit obtained is much the same over a wide temperature range, a temperature on the order of F. is ordinarily preferred as affording a relatively high rate of deposit under easily controlled conditions.

The solution may be contained in tanks lined with a variety of different materials such as stainless steel, rubber, lead, and certain ceramics.

One of the interesting features of the novel coating solution of this invention is its versatility in mode of application. Satisfactory deposits can be obtained byimmersion, brushing on, flow coating and spraying. "The last-named method is often especially advantageous since much of industry is geared to spray coat various types of materials for chemical surface treatment of metals as in phdsphating, chromating and the like. The mode of application Will generally be dictated by the facilities avail able and the size and shape of the articles to be coated. Insofar as we are informed, the deposition of a metal co'ating by chemical displacement from a spray application has never before been successfully achieved. Sound deposits having a thickness of 0.0003 inch are easily obtain'a'ble, and deposits having a thickness of from 0.005 to 0.00075 ineh may be obtained when employing properly adjusted solutions. Coatings of such thicknesses are airlple itec'tively to protect the underlying ferrous surface fi'bin emission, and considerably thinner coatings are often satisfactory for the purpose desired.

It will, of course, be appreciated that before the work is coated it should first be free of all oil, grease and dirt. Such prior cleaning operation may, for example, include vapor degreasing or preferably treatment with a chemical cleaner such as the commercially available alkali, emulsion or two-phase cleaners. Following such cleaning operation, the workshould be thoroughly rinsed in water and, if free of rust and scale, it need not be acid pickled. If pickling should be required, however, the Work will again be thoroughly water rinsed after completion of the pickling operation. It will then be conveyed to the processing tank or spray booth Where metallic lead is deposited thereon to the desired predetermined thickness. Subsequently, the work Will be rinsed first in cold water and then in hot water to facilitate drying, forced warm air drying being preferred.

The lead coated article may receive supplemental surface treatments such as chromating, oxalating, sulphating and phosphating in dilute solutions. Under proper conditions, the lead may be bright-flowed which further enhances its corrosion retarding ability. As a rule, howeyer, these various supplemental treatments will be found unnecessary. More frequently, the finished coated article will merely be painted or oiled, the surface being particularly suited therefor.

Corrosion and weathering tests have shown ferrous articles coated in accordance with the present invention to be exceptionally resistant to corrosion. Ferrous articles lead coated in accordance with this invention will 'Withstand 48 to 120 hours in the conventional salt spray test cabinet and, if oiled, considerably longer.

The employment of lead coatings to facilitate deep drawing operations and the drawing of wire, and parsent-any the drawing of steel wire, has been suggested since it has been determined that lead is a lubricant in such drawing operations superior to the usual greases, oils, soaps, emulsions, etc., but such other methods have been generally adopted in the industry for the reason that no economical method of applying the lead coating has previously been available. Thin lead coatings may also serve efiicientlyas a bearing metal in other applications.

The chemical components of our new lead coating bath may be mixed and milled together in dry form for shipment, it only being necessary to add the proper amount of water thereto at the point of use.

The aliphatic poly-basic hydroxy acids ordinarily employed to form the lead salt will of course normally be relatively low molecular weight poly-basic hydroxy fatty acids, that is to say, poly-carboxylic acids.

other modes of applying the principle of the invention may be employed, change being made as regards the de tails described, provided the features stated in any of the foll'dwing claims or the equivalent of such be employed.

We therefore particularly point out and distinctly claim asdiljif in entionz v,

1. The method of forir'ling a lead coating on a ferrous article which comprises applying thereto a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 4.5 to about 6.5, a lead ion concentration of from about 3 to about 50 grams per liter, and an operating temperature of from about F. to boiling for a time suflicient to deposit an appreciable lead coating on such article by a chemical displacement reaction.

2. The method of forming a lead coating on a ferrous article which comprises applying thereto a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 5.4 to about 6.0, a lead ion concentration of from about 10 to about 30 grams per liter, an operating temperature of from about 130 F. to about boiling, a small amount of an organic soundness-enhancing addition agent of the type commonly added to electroplating baths, and a solubilizing agent for such lead salt selected from the class consisting of the ammonium and alkali metal salts of aliphatic poly-basic hydroxy acids, for a time suflicient to deposit an appreciable lead coating on such article by a chemical displacement reaction.

3. The method of forming a lead coating on a ferrous article which comprises applying thereto a water solution of lead citrate having a pH of from about 5 .4 to about 6.0, a lead ion concentration of from about 10 to about 30 grams per liter, an operating temperature of from about 130 F. to about boiling, a small amount of an organic soundness-enhancing addition agent of the type commonly added to electroplating baths, and a solubilizing agent for such lead citrate selected from the class consisting of the ammonium and alkali citrates, for a time suffici'ent to deposit an appreciable lead coating on such article by a chemical displacement reaction.

4. The method of forming a lead coating on a ferrous article which comprises spraying such article with a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 4.5 to about 6.5, a lead ion concentration of from about 3 to about 50 grams per liter, and an operating temperature of from about 130 F. to boiling for a time sufficient to deposit an appreciable lead coating on such article by a chemical displacement reaction.

5. The method of forming a lead coating on a ferrous article which comprises applying thereto a water solution of lead citrate having a pH of from about 4.5 to about 6.5, a lead ion concentration of from about 3 to about 50 grams per liter and an operating temperature of from about 130 F. to boiling for a time suificient to deposit an appreciable lead coating on such article by a chemical displacement reaction.

6. The method of forming a lead coating on a ferrous article which comprises applying thereto a Water solution of lead citrate having a pH of from about 5.4 to about 6.0, a lead ion concentration of from about 10 to about 30 grams per liter and an operating temperature of from about 130 F. to about boiling for a time sufiicient to deposit an appreciable lead coating on such article by a chemical displacement reaction.

7. The method of forming a lead coating on a ferrous article which comprises applying thereto a water solution of lead citrate having a pH of from about 5.6 to about 5.8, a lead ion concentration on the order of about 15 grams per liter, an operating temperature of from about F. to about 210 F., a small amount of an organic soundness enhancing addition agent for inhibiting grain crystallization, and a solubilizing agent for said lead citrate selected from the class consisting of the ammonium and alkali metal salts of citric acid for a time suificient to deposit an appreciable lead coating on such article by a chemical displacement reaction.

(References on following page) 9 10 References Cited in the file of this patent 2,560,979 Pessel .Tuly 17, 1951 UNITED STATES 1) 2,658,842 Gutzeit NOV. 10, 433,074 Drautz July 29, 1890 FOREIGN PATENTS 570,554 Iordis Nov. 3. 1896 5 130,302 Great Britain of 1910 1,371,445 Shoemaker Mar. 15, 1921 2,111,199 Alvord Mar. 15, 1938 OTHER REFERENCES 2,2 0 02 Sullivan 4, 1941 Latimer, W. M. Oxidation Potentials, Prentice Hall,

2,236,296 Minich et a1. Mar. 25, 1941 Inc. pages 294-295. 

1. THE METHOD OF FORMING A LEAD COATING ON A FERROUS ARTICLE WHICH COMPRISES APPLYING THERETO A WATER SOLUTION OF A LEAD SALT OF AN ALIPHATIC POLY-BASIC HYDROXY ACID HAVING A PH OF FROM ABOUT4.5 TO ABOUT 6.5, A LEAD ION CON-CENTRATION OF FROM ABOUT 3 TO ABOUT 50 GRAMS PER LITER, AND AN OPERATING TEMPERATURE OF FROM ABOUT 130*F. TO BOILING FOR A TIME SUFFICIENT TO DEPOSIT AN APPRECIABLE LEAD COATING ON SUCH ARTICLE BY A CHEMICAL DISPLACEMENT REACTION. 